The present invention relates to optical devices such as LASER devices. More particularly, the present invention pertains to a method and apparatus for controlling temperature variations in a laser or other optical device.
Laser (Light Amplification by Stimulated Emission of Radiation) devices are well known in the art. These devices include semiconductor laser devices, which under the right circumstances will emit coherent light in a limited frequency range. An intrinsic part of the device is the optical resonator, including at least two opposing mirrors surrounding a gain medium. VCSELs typically employ distributed Bragg reflectors (DBRs) as the mirrors that bound quantum wells (QWs) that provide the gain. The DBR and QW structures are very thin epitaxial layers grown on a relatively thick substrate of near-lattice-matched semiconductor material.
Laser devices must be xe2x80x9cpumpedxe2x80x9d or excited in some way to achieve the conditions necessary for stimulated emission to occur. One method for pumping a semiconductor laser is to inject current through the device. Electrical contacts on the substrate and the DBR are designed to facilitate current flow through that region of the gain medium that subtends the optical aperture. Since electrical contacts typically do not have good optical transmission properties, a ring contact is often used on the exit aperture-side of the device to minimize optical loss for laser emission. A current field, and thus heating profile, is established between a ring annulus on one side of the device and a concentric plate on the other. This geometric arrangement of electrical interface causes the current to spread away from the optical axis, thereby competing with the ability to axially pump the quantum wells for fundamental transverse mode operation. In general, modal gain is maximized when the lateral current profile in the active region is well overlapped with the spatial mode(s) of the cavity one wishes to excite. Current-confining measures such as ion implantation and selective oxidation are often employed to increase the modal gain, thereby reducing threshold current and improving device efficiency. Several mechanisms are responsible for generating heat in VCSELs. Resistance to the flow of current through the DBR mirrors and substrate causes heating in these regions to be proportional to the square of the current [Pheat=I2R]. When current is injected into the quantum wells, electrons and holes recombine and release energy. Two recombination processes are possible: one is a radiative process responsible for the generation of photons, and the other is a nonradiative process resulting in heat. The amount of heat generated in this very thin region is linearly proportional to the injected current and voltage drop across the active layer [Pheatxcx9cIV]. Furthermore, the reabsorption of photonsxe2x80x94from both stimulated and spontaneous emission processesxe2x80x94can generate additional heating throughout the device.
All of these spatially-dependent heating processes lead the temperature of the laser device to increase rather non-uniformly. Inhomogeneous temperature fields not only lead to undesirable optical characteristicsxe2x80x94such as aberrations, self-focusing/defocusing, and specklexe2x80x94but make overall device performance current-dependent as well. Collectively, thermo-optic effects are responsible for limiting the useful output of many electro-optic devices to a very narrow operating range of current, and in some extreme cases, even a single-point design.
Temperature affects the propagation of light by modifying the refractive index of the medium through which the light travels. In general, light travels faster through a colder medium than a hotter medium due to the positive thermo-optic response, dn/dT (refractive index n increases with an increase in temperature T), characteristic of most optical materials. Therefore, a beam of light that passes through a medium with a lateral temperature gradient would experience a phase delay across the transverse dimension of the beam causing the wavefront to bend. This effect is sometimes referred to as thermal lensing because the temperature gradient works to focus (or defocus for materials with a negative dn/dT response) the light generated in the laser device. Thermal lensing is one of the fundamental considerations to the design of a single-mode laser cavity since the amount of lensing (expressed in terms of its effective focal length or inverse focal length) changes with pump power. As a result, single-mode laser action tends to be restricted to a relatively narrow range of applied current.
One known method for limiting the temperature rise in a laser device is to physically couple it to a heat sink. The maximum temperature rise and thermal resistance for the device scales inversely with the area A and thermal conductivity k of the heat sink. It is common practice in thermal design to maximize both of these variables within the typical constraints of manufacturability, cost, package size, availability of materials, etc. When the same methodology is applied to the heat transfer in VCSELs, radial temperature gradients in the device (relative to the light beam) not only prevail but are exacerbated. As a result, thermal lensing effects can even plague a laser device""s ability to exhibit low thermal resistances.
In view of the above, there is a need for an improved method and apparatus to control temperature variations in a laser device.
In an embodiment of the invention, a heat sink is coupled to a first portion of a surface of a light-emitting device via a heat sink interface, while an insulating region is disposed between a second portion of the surface of the light-emitting device and the heat sink. In another embodiment of the invention, a method is provided for controlling a temperature distribution profile in an optical device. In other embodiments of the invention, a reduced strain optical device, and a method for reducing strain in an optical device are provided.